A color CRT employs an apertured shadow mask disposed a predetermined distance from the CRT's display screen which serves as a color selection electrode by shadowing, or blocking, all but selected ones of the individual light-emitting phosphors on the display screen from its corresponding electron beam-emitting source located in the neck of the CRT. By blocking two of the electron beams while permitting the third to be incident upon its designated light-emitting phosphor elements, a video image comprised of the three primary colors red, green and blue is presented on the CRT's display screen. The shadow mask has a very large number of electron beam passing apertures and is in the form of a thin sheet of a high strength metal. The shadow mask is maintained in a stretched condition under tension and is typically provided with a slight curvature. For example, in a typical 14-inch color CRT, the shadow mask is provided with a curvature of 1R.
The shadow mask is disposed within and attached to a peripheral frame. Disposed about the shadow mask frame and attached to respective upper, lower and lateral portions of the shadow mask frame are typically four resilient metal holders, or springs. During CRT operation, the shadow mask is heated by the electron beams incident thereon. To compensate for the thermal related shadow mask aperture shift relative to the display screen's phosphor elements, prior art approaches have employed bimetallic shadow mask holders which incorporate a first metal having a high coefficient of thermal expansion in side-by-side combination with a second metal having a low coefficient of thermal expansion with various surface area ratios. This type of shadow mask installation compensates for shadow mask aperture shift caused by mask doming by moving the mask/frame assembly along the longitudinal axis (Z-axis) of the CRT toward the CRT's display screen. Some of this heat is transferred to the metal holders maintaining the shadow mask in position within the CRT glass envelope. Heating of the resilient metal holders causes the metal holders to also undergo thermal expansion causing the four resilient metal holders to rotationally displace the shadow mask in the X-Y plane of the shadow mask about the longitudinal axis of the CRT. Rotation of the shadow mask is due to the axis of deformation of the four metal holders which is along their respective longitudinal axes. Shadow mask rotation also results in misregistration of the shadow mask apertures with the display screen's phosphor elements causing a degradation of video image color purity. The resilient metal holders are typically of the bi-metal type and are used in combination with a shadow mask typically comprised of aluminum killed (AK) steel. Problems of electron beam landing misregistration and color purity degradation become even more severe in the case of shadow masks with finer aperture pitch and flatter configurations as commonly encountered in larger CRTs. In larger CRTs employing flatter shadow masks, e.g., 15" CRTs and larger, where the aluminum killed (AK) steel shadow mask can no longer provide the required electron beam landing registration requirements, an Invar shadow mask is generally used, with the bi-metallic shadow mask holders typically replaced with a single-metal holders comprised of SS-302 or SS-320 stainless steel because there is now no need to compensate along the Z-axis. However, the shadow mask rotation problem still remains with these stainless steel shadow mask holders because of their relatively high coefficient of thermal expansion. The problem of electron beam misregistration becomes even more severe in the case of shadow masks with finer aperture pitch and flatter configurations as commonly encountered in larger CRTs.
The present invention addresses the aforementioned limitations of the prior art by providing a shadow mask mounting arrangement employing materials having a very low coefficient of thermal expansion, i.e., preferably below 2.0.times.10.sup.-6 cm/cm/.degree. C. Representative shadow mask holder materials include Invar, Neovar and Kovar. Because of the very low coefficient of thermal expansion of these materials, directional rotation of the shadow mask during CRT operation is substantially reduced for improved video image color purity.